An optical fiber laser generally includes an optically active cavity defined in an optical fiber, and reflective means for partially confining electromagnetic radiation within the cavity. By "optically active" is meant that at least a portion of the cavity is doped with a distribution of ions or atoms capable of exhibiting stimulated emission at the desired laser wavelength, when pumped by electromagnetic radiation at a (generally shorter) pump wavelength or range of pump wavelengths. In silica-based optical fibers (i.e., optical fibers having a core which comprises at least 80% silicon dioxide), useful dopants for this purpose are rare-earth ions such as Er.sup.+3. The reflective means are conveniently provided in the form of at least one, and more typically two, distributed Bragg reflectors (DBRs). DBRs are exemplarily created by exposing an optical fiber having at least some photosensitivity to ultraviolet radiation of an effective wavelength for producing refractive index changes in the fiber. A periodic pattern is imposed on the impinging radiation by, e.g., superimposing a pair of beams of substantially monochromatic radiation to create an interference pattern. When such a patterned radiation field impinges on an optical fiber of the appropriate photosensitivity, a corresponding pattern is imposed on the core of the fiber in the form of periodic (or quasiperiodic) fluctuations in the core refractive index. A technique for creating such reflectors is described in U.S. Pat. No. 4,725,110, issued to W. H. Glenn, et al. on Feb. 16, 1988, and U.S. Pat. No. 4,807,950, issued to W. H. Glenn, et al. on Feb. 28, 1989. An optical filter which comprises a Bragg grating formed in an optical fiber is described in U.S. Pat. No. 5,007,705, issued to W. W. Morey, et al. on Apr. 16, 1991.
Each DBR functions as a wavelength-selective reflector having a reflectance curve (as a function of wavelength) having at least one well-defined peak. The precise wavelength of operation of the laser is determined, at least in part, by the relationship between the modal structure of the cavity and the reflectance curve. That is, for the laser to exhibit gain at a given wavelength (under appropriate stimulation), the given wavelength must not only fall within a reflectance peak, but must also correspond to a Fabry-Perot resonance (i.e., a mode) of the laser cavity.
As is well-known in the art, the spacing between wavelengths corresponding to successive modes increases as the length of the cavity decreases. As a consequence, reducing the cavity length may tend to confine the laser gain to a few modes, or even a single mode. This can result in a laser which has high mode stability. Mode stability is advantageous when, e.g., a source of laser radiation of a precisely defined wavelength is desired.
However, a substantial amount of gain can be preserved in a shortened cavity only by concomitantly increasing the doping level. The doping level cannot be increased indefinitely. One limitation on the doping level is the tendency of dopant ions, such as erbium ions, to aggregate at high concentrations, a phenomenon sometimes referred to as "clustering". Clustering has been associated with parasitic deactivation effects which reduce the efficiency of the laser. We have discovered that, quite surprisingly, a doping level sufficient to impart useful gain to a laser cavity 5 cm long or less can be achieved without suffering an intolerable amount of parasitic loss.
An optical fiber laser having a DBR-terminated cavity is described, e.g., in G. A. Ball and W. W. Morey, "Continuously tunable single-mode erbium fiber laser", Optics Lett. 17 (1992) 420-422. Described therein is a single-mode, standing-wave, fiber laser fabricated in germanosilicate fiber doped with erbium to approximately 550 parts per million (ppm). Interfering ultraviolet beams were used to write a pair of DBRs spaced 10 cm apart. The resulting laser was reported to have an optical output power of 100 .mu.W and a slope efficiency of about 0.25%.
Although successful laser operation has been reported in fiber cavities of 10-cm length or more, practitioners in the field have hitherto failed to provide a useful fiber laser having a substantially shorter cavity, such as a 1-cm cavity. Such a laser is desirable because it can potentially offer enhanced mode stability, as discussed above. Such a short-cavity laser is also desirable because it can facilitate the incorporation of fiber lasers with semiconductor pump lasers in compact packages. Still further, such a laser is desirable because its compact nature reduces its susceptibility to temperature fluctuations and mechanical perturbations.